There are many commercial settings (e.g., factories, stores, warehouses, etc) that have a large number of lighting fixtures with installed fluorescent tubes (e.g., T8 or T12 tubes) and accompanying electronic ballasts.
It should be noted that electronic ballasts differ from traditional electromagnetic (EM) ballasts in a number of ways. EM ballasts operate at line frequency (e.g., 50 Hz or 60 Hz), while electronic ballasts operate at significantly higher frequencies (e.g., 40 kHz), thereby reducing visible flicker. Electronic ballasts are also typically more efficient than EM ballasts, smaller and lighter than EM ballasts, and sometimes also provide galvanic isolation between their inputs and outputs, while EM ballasts typically do not. On the other hand electronic ballasts are more complex and require more electrical circuitry than EM ballasts.
Illumination devices based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, longer expected lifetime, lower operating costs, and many others.
Accordingly, in some cases there is a desire to replace existing traditional fluorescent light sources with newer, more energy-efficient LED light sources. In particular, in many installations it would be desirable to replace installed fluorescent tube lamps with LED light sources.
There are various LED replacement tube lamps being sold at the market today. In order to operate such LED lamp properly, most of them require rewiring of the existing fluorescent tube lamp lighting fixture, either by removing the existing electronic ballast or by bypassing it. However, this is labor-intensive and increases the total replacement cost. The easiest way to upgrade an existing fluorescent tube lighting unit with an LED light source is by simply removing the old fluorescent tube lamp and replacing it with a new LED tube (TLED) lamp. No rewiring or opening of the fluorescent tube lamp lighting fixture should be required.
Also, one of the major challenges of TLED lamp is to reduce costs. LED driver cost can be a significant part of the total lighting unit, particularly if a switching mode power supply (SMPS) driver is used. Operating with an existing electronic ballast presents an opportunity for reducing the cost of a TLED lamp by utilizing the existing electronic ballast as part of the LED driver system.
Furthermore, energy savings is one of the key advantages for TLED lamp. Because a TLED lamp typically shines downwards (i.e., the light exit window is typically between 180 to 240 degrees) instead of all around the tube (a fluorescent tube lamp has a light exit window of 360 degree by definition), a TLED lamp can provide a significantly higher optical efficiency than a fluorescent tube lamp. Therefore, the power consumption of a TLED lamp will be lower than a corresponding fluorescent tube lamp. For example, to replace a 36 W 4 ft TL lamp, power consumption of a TLED lamp is typically specified at about 20 W. Therefore, power reduction of over 40% needs to be realized by the LED driver.
However, an installed electronic ballast in an existing fluorescent tube lamp lighting fixture is intended to operate a fluorescent lamp. A standard 4 foot T8 fluorescent tube lamp is rated at 32 W power with an electronic ballast and the ballast is designed to drive the TL lamp with the correct current for this power level. In the simplest case, an LED string could be connected to the electronic ballast with the help of a diode bridge for converting the high frequency AC power supplied by the electronic ballast to the DC current for the LED string, with a capacitor for smoothing out the ripple current. If the LED string is chosen substantially close to a burning voltage of the TL lamp, the LED lamp will receive similar power and current compared to the TL lamp.
However, this simple method does not have closed loop power/current control of the TLED lamp. Therefore the amount of light output would be heavily impacted by the different electronic ballast circuit designs, fixture lamp configurations (i.e., single, dual lamp), mains voltage variation, and the spread of the LED forward voltage together with its temperature dependency. It is very difficult for such circuit to achieve LED current tolerance of +/−10%.
That is, one of the challenges of providing a TLED lamp for retrofit into existing fluorescent tube lamp lighting fixtures with electronic ballasts is the provision of an LED lighting driver which can reliably operate with a variety of existing installed electronic ballasts designed for fluorescent tube lamps, and with LEDs having a reasonable variation on forward voltage.
In summary, in order to eliminate labor costs associated with installing new lighting fixtures or rewiring existing lighting fixtures, it would be desirable to retrofit LED tube (TLED) lamps into the existing fluorescent tube lamp lighting fixtures having electronic ballasts, in place of the existing fluorescent tube lamps, while leaving the existing electronic ballasts in place. Additionally, it would be desirable to be able to retrofit the same TLED lamp into different lighting fixtures having a variety of different electronic ballasts with different output levels. Furthermore, it would be desirable to provide a TLED lamp design which can operate with a variety of different electronic ballasts with different output levels over a range of tolerances for the LEDs themselves.